A known braking system used commonly in civil aircraft is shown in FIG. 1. The braking is commanded by either the pilots' brake pedals 1 or by an autobraking control unit 2. When autobraking is in use (i.e. autobraking is selected and not being overridden by the pilots' commands), the autobraking unit provides a command signal 3 to the braking system that is constant. This provides a level of deceleration from the brakes that, while it is intended to be constant, varies significantly for many reasons. The autobraking control unit 2 provides several levels of landing braking strength (marked LO,2,3,4,HI in the Figure) and a strongest level (marked RTO) for a rejected take-off, each braking level having higher level of command signal 3.
The command signal operates a hydraulic servo valve 4, which operates the brakes, determining the brake pressure 5 applied to the brakes. A feedback circuit is used to adjust the pressure applied to a particular wheel. The wheel speed 6 (in the form of the speed of the rim) is measured by a tachometer and is subtracted from a reference speed 7. The resultant speed error 8 is processed by an anti-skid filter 9, and the resultant signal is subtracted from the command current before it is applied to the servo valve 4.
The purpose of the feedback circuit is to stop the wheels 10 from skidding (and consequently locking). If the speed error is large that is because the wheel is slipping more than is desirable and this results in the feedback signal reducing the commanded current and hence the brake pressure is reduced and the skid is eliminated. The reference speed is generated to check the level of skidding that may have occurred. There are several methods in use for calculating the reference speed. Two of them are described here.
The first method is to calculate the reference speed continuously as (1−λOPT)·vGROUND, where vGROUND is the ground speed (measured for example by the aircraft's inertial guidance systems) and λOPT is a predetermined constant, namely the “optimum” slip ratio, i.e. the slip ratio for which the friction coefficient between tyre and ground is maximum. (The slip ratio is one minus the ratio between the speed of the tyre at its edge and the ground speed.)
In the second “adapting” method, when a wheel's angular deceleration is below a certain threshold, a skid is detected and the slip ratio is reduced (by the controller) by a step increase of reference speed, which generates a large speed error, thus immediately reducing the brake pressure. This controls the braking to occur around the actual optimum slip ratio rather than a predetermined “optimum” slip ratio value. This method suffers from the disadvantage that it uses tachometers to measure angular velocity, which are not that accurate.
The uneven deceleration provided by this known system is a problem. In particular the inventor has realised that it can cause the attitude of the aircraft to change thus exciting the modes of the structure of the aircraft. These modes include both the normal modes of flexing of the structure of the aircraft and oscillations in the attitude of the aircraft, i.e. modes of rigid rotation of the aircraft. The flexing of the aircraft structures may cause undesirable fatigue damage. Excitation of the modes also causes an uncomfortable ride for the passengers.
There are several sources of the uneven deceleration.
One source is the braking provided by the ground wheel brakes of the landing gears. The braking force provided by a ground wheel is equal to μFz where μ is the coefficient of friction between the wheel and the ground and Fz is the vertical load on the wheel. So, variation in the vertical load on a wheel leads to variation in the braking force. This variation in braking force in turn excites the modes of the aircraft (rigid rotational modes and normal modes of flexing), to which, of course, the wheels providing the braking are attached. In turn the motion of the aircraft due to these excited modes affects the vertical load on the wheels, which can further excite the modes, and so on.
Also, the coefficient of friction can vary along a runway, and from runway to runway and from time to time, e.g. by being wet or dry. This therefore varies the braking force, again exciting the modes of the aircraft.
Another factor is that the known braking systems often oscillate between braking (stable behaviour) and skidding (unstable behaviour) and the wheels running freely: when the wheels start to skid, the brake control system then prevents the skid. As the demand for braking is of course high, skids may soon reoccur. This switching back and forth changes the attitude of the aircraft and thus is another cause of excitation of the modes of the aircraft.
Another source of unevenness of braking in the known system is that the aircraft also undergoes the braking caused by aerodynamic drag. The magnitude of this is related to the square of the speed of the aircraft and so the initial contribution to braking is great but towards the end of braking is only a minor contribution.